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I've played around with a few water heaters in attics that have one or more PAVs.
On the ones I checked out, the draft hood provided enough of an exit point for the air being pulled down the over fire draft in the water heater wasn't reduced all that much.
Under the right conditions, like in a case of a severe backdraft, I can imagine the velocity of the air coming down the vent pipe being high enough to stop the over fire draft and cause flame rollout though.

One house I found had 3 PAVs, 3 water heaters and 3 furnaces in the attic, with absolutely no soffit ventilation. The water heater vent pipes were backdrafting bigtime, the airflow was actually clearly audible, but the water heaters still had enough over fire draft to work. The 80% furnaces were backdrafting so bad that the draft inducers couldn't close the pressure switches though, lol.
The house itself was being pulled into a 24 Pa negative pressure!

I suppose, then, that the PAV's were pulling the stack gasses out through the draft hoods, and since the house was 24 Pa negative to atmosphere, there was little risk of stack gasses entering the house...correct?

Even so, what in your mind would create a backdraft severe enough for rollout to occur with PAV's in operation? A sudden gust of wind with the stack on the windward side of the roof?

I had a separate thought today regarding soffit venting/ridge venting or whirlybird venting near the peak of the roof. I have stood under soffit vents on hot days and felt hot air spilling out of them. I can't recall at the moment how windy the day was when I observed this, but probably not all that high. The house in my memory where I last observed this occurring has a hip roof with soffit vents all around, and whirlybirds near the peak.

I see two causes, and your experience can either confirm or nullify my thinking...

With little to no wind, the heated air in the attic expands to the point where it drops out the soffit vents. To do this it would need to draw from the whirlybirds or ridge venting, or from the house interior, unless it merely dumps at any available opening due to sheer expansion. Regarding the house interior, being that it is cooler and the air is more dense (hence a slightly lower pressure), the "reverse stack effect" through ceiling penetrations would conceivably allow attic air to enter the house. The point of make-up air for the attic air as it heats, expands, and dumps out the soffit vents would need to come from somewhere.

With adequate wind pushing passing over the roof, air could conceivably flow down ridge venting or whirlybird openings and then pressurize the attic, hence causing heated air to dump out the soffit vents. But this same air is also pushing against windward facing walls, whereby air would be encouraged to flow into soffit vents due to pressure difference between windward wall and attic conditions. Hence, it may cause hot air to dump out of soffit vents on the leeward side of the house, and any roof venting on the leeward side of the roof.

I had a separate thought today regarding soffit venting/ridge venting or whirlybird venting near the peak of the roof. I have stood under soffit vents on hot days and felt hot air spilling out of them. I can't recall at the moment how windy the day was when I observed this, but probably not all that high. The house in my memory where I last observed this occurring has a hip roof with soffit vents all around, and whirlybirds near the peak.

I see two causes, and your experience can either confirm or nullify my thinking...

With little to no wind, the heated air in the attic expands to the point where it drops out the soffit vents. To do this it would need to draw from the whirlybirds or ridge venting, or from the house interior, unless it merely dumps at any available opening due to sheer expansion. Regarding the house interior, being that it is cooler and the air is more dense (hence a slightly lower pressure), the "reverse stack effect" through ceiling penetrations would conceivably allow attic air to enter the house. The point of make-up air for the attic air as it heats, expands, and dumps out the soffit vents would need to come from somewhere.

With adequate wind pushing passing over the roof, air could conceivably flow down ridge venting or whirlybird openings and then pressurize the attic, hence causing heated air to dump out the soffit vents. But this same air is also pushing against windward facing walls, whereby air would be encouraged to flow into soffit vents due to pressure difference between windward wall and attic conditions. Hence, it may cause hot air to dump out of soffit vents on the leeward side of the house, and any roof venting on the leeward side of the roof.

Food for thought, yes?

When the wind blows, even a little, the air pressure on the side of the house the wind is blowing on will be higher, and the pressure lower on the oposite side, so you can/will have air blowing in the vents on one side, and air coming out of any vents on the lower pressure side of the house.

An attic with good passive ventilation will usually have quite a bit of air moving through it on a windy day. With all the wind we have been having this summer, most of the attic work I've been doing hasn't been so bad.

I just put one of these ventilators in and I'm about to remove it and return it to the big box store to find another. The carton claimed "made in USA". The mounting frame and support are all plastic. Response to email inquiry about country or origin for this "USA made" ventilator was that various parts "imported". They won't answer who made the thermally protected motor. It will be out of here the next trip up the road.

I think it is reckless engineering practice to mount a motor that can seize and burn in plastic, regardless of the overload protection. Plastics have not performed well in Ford intake manifolds where vibration was an issue. A cracked ventilator mount is likely to drop the motor into the attic and lock the motor to burn.

I want to know how "Made in USA" printed on the carton turns into "imported/various" when directly inquired about slovenly engineering practice of a motor that can burn in your attic being supported in plastic.

Shop carefully. Open the box and look at the contents. It will save you an extra trip down the road.

Originally Posted by southerncomfort

I generally dont like attic ventilators because most of them employ a shaded pole motor which has the least efficiency of any electric motor.
Usually they dont last for more than a summer of two.

I would use some type of temp probe "type K" to get your temp readings
The infared hand held thermometers are a joke as far as i am concerned

you are very welcome OP.
but I have to admit that I lost the link...and some kind soul posted it
...seems we get LOTS of pav issues here.

I can't remember if it was florida solar energy center or building science
that proved that radiant barriers raised shingle temp 2 to 3 degrees.
But while shingle mfg may pay for bundles of shingles...the tearoff and install
comes from ho's pocket.
Not that I have seen any shingle mfg's that stood behind their warranty...

Good thread...I'll have to re-read when I am not so tired. Those attic temps
really can drain me. I like to work in attics from 6 am til around noon.

keep it going folks!
And have a great day tomorrow...weekend is almost upon us.

PS...good to read your inputs, I like the way you investigate.

The cure of the part should not be attempted without the cure of the whole. ~Plato